The quantum mind theory is a belief that large-scale quantum coherence
is necessary to understand the mind and brain.

This is very much a minority opinion, although it does have the support
of the well-known mathematical physicist Roger Penrose. Other proponents
include Stuart Hameroff, Subhash Kak, Karl Pribram, and Henry Stapp. The
argument for the quantum mind is that classical mechanics cannot explain
consciousness.

It also has the support of Brian Flanagan and Michael Lockwood.

The main argument against the quantum mind is that the brain is warm,
wet and noisy and that the structures of the brain are much too large
for quantum mechanics to be important. Consequently, it is difficult for
coherent quantum states to form for very long in the brain, and impossible
for them to exist at scales on the order of the size of neurons. These
issues have led Penrose to argue that consciousness is not a consequence
of interactions between neurons in the brain but arises as from microtubules
within cells, which are much smaller and for which quantum effects could
be significant. This was originally the theory of Stuart Hameroff.

On the other hand, a system does not cease to be quantum because it is
wet and noisy. And then, what was previously dismissed as "noise"
in the brain has recently been discovered to be complex signals.

Then again, if the brain is fractal in character, it may well exhibit
sensitive dependence on initial (quantum) conditions. Given the fractal
character of dendritic arborizations, brain function may depend on self-similar
processes at lower spatio-temporal scales. Or, neural form follows quantum
function. If all matter consists of quantum fields, as Dyson makes explicit
in his Scientific American article on "Field Theory," (reference?)
then the brain is a collection of such fields.

This view is very different from conventional views of how the brain
works, in which neurons communicate via electric impulses which trigger
the release of neurotransmitters in the synapses. In the conventional
view of brain function, microtubules play no significant role in brain
function other than to provide structural support to the neurons. The
theory of the quantum mind has been criticized on a number of grounds.
For one, it fails to explain how chemicals and physical processes which
affect neuron functioning would cause generally predictable changes in
consciousness, whereas the conventional theory provides an explanation
for how psychoactive substances work and how the brain would react to
injury.

Lockwood, Stapp, and Flanagan have argued that the rich state space of
quantum theory easily accommodates the variety of perceptual state spaces,
whereas classical theory cannot.

Also, decoherence mechanisms such as emission of thermal radiation appear
to apply to large molecules such as microtubule protein subunits and synaptic
vesicle proteins, making quantum coherence on the scale proposed for quantum
mind theories unlikely.

Quantum theories of mind are among the few classes of theories acceptable
in the philosophical stances of pseudonomenalism and mind/brain identity
theory.

There is another type of quantum theory of mind called the many-minds
interpretation that is invoked as a conservative version of the many-worlds
interpretation of quantum theory and does not involve collapse of the
QM wave function.